I have, in more than thirty years of performing onstage, had many, many strings break. Other than an elevated heart rate from the sound, I don't recall ever having experienced any adverse physical effects. My daughter was once clipped by a mandolin string which broke during one of her solo breaks, but she was able to finish the set, and had beyond a slight welt where the string had popped her. Many years ago, I was present onstage when a well-known bluegrass banjoist actually cut the strings off the banjo of another musician as he was playing. It was done in fun, and there were no injuries.